1. Field of the Invention
The present invention relates to workpiece inspection apparatus and method and also to a software program for causing computers to execute the method. More particularly but not exclusively, this invention relates to pattern inspection technologies for inspection of pattern defects of a test object, such as a workpiece in the manufacture of semiconductor devices. The invention also relates to apparatus for inspecting ultrafine pattern defects of photomasks, wafers, liquid crystal substrates or else for use in fabrication of semiconductor devices and liquid crystal display (LCD) panels.
2. Description of the Related Art
In recent years, with the quest for higher integration and larger capacity of large-scale integrated (LSI) circuits, semiconductor devices are becoming narrower in circuit linewidth required. These semiconductor devices are fabricated by using an original or “master” plate with a circuit pattern formed thereon (also called a photomask or a reticle as will be generically referred to as a mask hereinafter) in a way such that the pattern is exposure-transferred by reduced projection exposure equipment, known as a stepper, onto a target wafer to thereby form thereon a circuit. For the manufacture of a mask to be used to transfer such ultrafine circuit pattern onto wafers, pattern photolithography equipment is used, which is capable of “drawing” microcircuit patterns.
Improving manufacturing yields is inevitable for the microfabrication of LSI chips which entail increased production costs. Currently, circuit patterns of LSIs, such as 1-gigabit class dynamic random access memories (DRAMs), are on the order of nanometers (nm), rather than submicron order. One major factor for reducing yields is the accuracy of the apparatus for detecting defects, which take place in a mask pattern as used when an ultrafine pattern is exposed and transferred onto semiconductor wafers by photolithography techniques. As LSI patterns to be formed on semiconductor wafers are further miniaturized in recent years, the size dimensions that must be detected as pattern defects became much smaller than ever before. Thus, a need is felt to achieve further increased accuracy of the pattern inspection apparatus operable to inspect the mask for defects.
Incidentally, with recent advances in multimedia technologies, LCD panels are becoming larger in substrate size and finer in pattern of thin film transistors (TFTs) as formed on liquid crystal substrates. This larger/finer trend requires an ability to inspect ultrasmall pattern defects in a wide range. For this reason, it is an urgent challenge to develop an advanced workpiece inspection apparatus capable of efficiently inspecting defects of photomasks in a short time period, which are for use in the manufacture of such large-area LCD patterns and large-screen LCD panels.
An ordinary approach to performing inspection in prior known pattern inspection apparatus is to compare an optical image resulted from the image sensing of a pattern formed on a workpiece such as a mask at a specified magnification to design data or, alternatively, compare it to a sensed optical image of an identical pattern on the workpiece in a way as disclosed, for example, in Published Japanese Patent Application No. 8-76359 (“JP-A-8-76359”). An example of pattern inspection methodology is the so-called “die to die” inspection method for comparing optical image data obtained by image pickup of identical patterns at different locations on the same mask. Another example is a “die to database” inspection method having the steps of receiving computer-aided design (CAD) data indicative of a designed pattern, converting the CAD data into graphics data (i.e., design pattern data) with an appropriate format for input to photolithography equipment, inputting the data to an inspection apparatus, generating design image data (reference image data) based on the input data, and then comparing it to optical image data, that is, measurement data resulting from the image pickup of a target pattern being tested. The inspection method for use in such apparatus, the workpiece is mounted on a stage, which moves to permit light rays to scan a surface of the workpiece for execution of the intended inspection. A light source and its associated illumination optical lens assembly are used to emit and guide the light to fall onto the workpiece. The light that passed through the workpiece or reflected therefrom travels via the optics to enter a sensor so that a focussed optical image is formed thereon. This optical image is sensed by the sensor and then converted to electrical measurement data, which will be sent to a comparator circuit. After position-alignment between images, the comparator circuit compares the measured data to reference image data in accordance with an adequate algorithm. If these fail to be matched, then determine that pattern defects are present.
The linewidth of design pattern data becomes finer in recent years. In addition, due to the presence of micropatterns for the optical proximity correction (OPC) use, it becomes more difficult to match together the design image data and the optical image data for use as measured data. This difficulty can often lead to inspection errors—that is, those that are inherently not judged as defects are erroneously regarded as defects, known as false or “pseudo” defects. One approach to avoiding this problem is to “loosen” a decision threshold as used in the comparator circuit. Unfortunately, this approach does not come without accompanying a penalty which follows: the to-be-detected size accuracy is lowered, resulting in that any defects in the required pattern are no longer detectable. Thus it is required to apply comparison inspection to the “imaged” pattern at certain level of inspection accuracy as selected from a plurality of predefined ranks of accuracy on a case-by-case basis.
A technique for performing the comparison inspection while categorizing graphics patterns into a plurality of ranks is disclosed, for example, in JP-A-2004-191957 and JP-A-10-142771. However, these Japanese patent documents fail to teach any practically implementable scheme for categorizing graphic patterns in multiple ranks to enable realization in the apparatus, which is deemed impractical and deficient from a viewpoint of practicability. Thus it is demanded to attain a solving technique thereof.
It is an ordinary approach that in case defects are found in the workpiece of interest, defect reviewing is carried out by a user. However, when the above-described OPC-based micropatterns are diversified, inherently defect-free patterns can be misjudged as defective ones, causing user-executed defect review workload to go beyond the limit in terms of the time required. This in turn poses a problem as to redoing of the inspection per se in cases where a large number of pseudo-defects, such as those stated above, appear within the workpiece. Alternatively, a problem arises as to a need to prepare again the high-priced workpiece itself. Adversely, the decision threshold is loosened, there was a drawback concerning the lack of an ability to detect defects in a pattern which is under strict size accuracy requirements. Furthermore, from viewpoints of avoiding unwanted increases in scale and complexity of inspection equipment along with cost rise-up and development period prolongation, a need is also felt to minimize amelioration of the currently existing inspection apparatus for overcoming the problems stated above.